1. Field of the Invention
The present invention relates to automotive mirror assemblies, and more specifically, to a heatable external rearview mirror for motor vehicles that includes a thermochromic heating indicator for detecting heating of the mirror pane by a heating element and providing a visual indication of the operation of the heating element.
2. Description of Related Art
To design external rearview mirrors for motor vehicles so as to be heatable is known, so that condensate, frost, snow, ice, etc. may be removed by heating the mirror pane by means of a heating element in order to obtain an unobstructed field of vision to the rear through the mirror. Typically, the heating elements are made of surface heating elements extending essentially all across the expanse of the mirror pane at its back side with a mirror housing. It is also known to actuate the heating element in heatable rearview mirrors by means of a thermostatic switch, i.e. that the thermostatic switch closes the power circuit going to the heating element so as to actuate it when a predetermined outside temperature has been reached.
As a result of the mirror heating system or the heating element being switched on and off automatically by the thermostatic switch, the driver of a vehicle equipped with one or several of these mirrors may often find it difficult to recognize whether the mirror heating system has already been automatically switched on or not. What often occurs is that the mirror pane becomes “blind” due to condensation, etc., while the thermostatic switch has however not yet switched on because the temperature for this may not yet be low enough for it. Also, the thermostatic switch may have switched on just before checking the visibility of the mirror but no result is yet visible in the short time since the moment of being switched on because the mirror pane could not yet be heated up sufficiently. In such cases the mirror heating system is then usually switched on manually by the driver, but this additional action by the driver is often unnecessary in instances when the thermostatic switch has switched on just before checking the visibility conditions of the mirror, while no results are yet visible because the mirror pane could not yet be heated up sufficiently.
For the driver of a motor vehicle equipped with one or several heatable rearview mirrors it is therefore desirable to know whether the automatic mirror heating system on a condensation-covered or “blind” mirror has already switched on for the removal of this condensation or not, so that he may override the thermostatic switch if necessary and switch on the heating element by means of a separate switch. Especially on commercial vehicles it is also customary for the mirror heating system to be actuated and de-actuated only manually.
It is known to provide optical heating indicators to avoid overheating of the mirror surface and any resulting damage to the mirror or injury to persons touching the overheated mirror. In this context DE 19922778 discloses the installation of a heating indicator in the form of an LED within the heating circuit that is visible in the mirror. In this manner the operation of the mirror heating system is displayed optically. The disadvantage of this is that the LED must be built into the heating circuit. Because of different heating currents when using PTC elements as heating system, this results in excessively different influences on the LED and thereby different degrees of lightness. In addition, this LED does not indicate the effectiveness of the mirror heating system but merely its operation. In cases of a short-circuit in the surface heating element, the LED lights up even thought the functionality of the mirror heating system is insufficient. In addition, retrofitting existing mirror systems with such technology is difficult.
It is therefore an object of the present invention to create a heatable external mirror with a heating indicator, in particular a heatable rearview mirror for motor vehicles which displays the function and effectiveness of the mirror heating system.